Electron multiplier tube circuits



l Dec. 28, 1948. M H. vSWEET 2,457,747

ELECTRON MULTIPLIER TUBE CIRCUITS Filed Feb. 15. 1946 l-Natented Dec. 28, 1948 ELECTRON MULTIPLIER. TUBE CIRCUITS Monroe H. Sweet, Binghamton, N. Y.,'assignor to General Aniline & Film Corporation, New York,

N. Y., a corporation of Delaware Application February 15, 1946, Serial-No. 647,932

(Cl. Z50-Z7) l 20 Claims.

This invention relates to electron multiplier tube circuits, and more particularly to such circuits incorporating novel stabilizing means.

v"I'he present application is a continuation-inpart of my copending application Serial No. 570,627 iiled December 30, 1944, for Logarithmic photometers.

In the above identledapplication, there are shown and described logarithmically responsive measuring circuits incorporating electron multiplier tubes. These circuits include feed-back arrangements wherein the potentials applied to the elements of the multiplier tube are made of the tube. Specifically, the output current of the multiplier tube is applied to the control grid of an electronic amplifier tube which controls the potential drop across a potentiometer having equi-spaced taps connected to the elements of the multiplier tube.

In the circuits of said copending application, the voltages for all of the dynodes, or intermediate electrodes, of the multiplier tube and for the anode are derived from the equally space taps on the bleeder resistor or`potentiometer connected in the output circuit of a pentode control tube having its output connected in series with the bleeder resistor and a source of potential.

rThe bias applied to the pentode tube control the level of illumination incident.. on the photo,.-I

surface of the multiplier tube is low, the bleeder current will be high and the voltage between the anode and last dynode will be correspondingly high. Consequently, nearly all of the electrons emitted from the last dynode are collected by the anode, and the full current of which the multiplier tube is capable for that particular light level and applied dynode voltages will be utilized.

However, athigher levels of incident illurni-I nation on the multiplier tube, the bleeder current, and consequently the dynode voltages land the anode voltage, are all quite low. Under these circumstances, a small change in anode voltage will effect a very large change in anode current. This results in relatively unstable operation of the multiplier tube over the range of operating voltages.

ydependent upon the anode or output current It is among the objects of this invention to provide a stabilizing circuit for an electron multiplier tube; to provide an electron multiplier tube circuit in which the voltage between the last dynode and the anode of the multiplier tube is held at a substantially constant value, of a reasonably high magnitude; to provide a logarithmically responsive measuring circuit, including a multiplier tube and bleeder current feedback means controlling' the operating potentials of the dynodes of the multiplier tube, but in which the potential between the last dynode and the anode of the multiplier tube .is substantially independent of the bleeder current; to provide suchl a measuring circuit including means forminimizing the'eifects of small changes in the grid voltage-plate current relation of an electronic tube controlling the bleeder feed-back current; to provide such a measuring circuit which is completely independent of all ordinary variations in supply line voltage; to provide such a measuring circuit in which calibration is not effected by changes in the characteristics of the electronic tube controlling the bleeder current feed-back means; and to provide such a measuring circuit having increased overall stability of performance.

These and other objects, advantages and novel features of the invention will be apparent from the following description and the acompanying drawing. In the drawing:

Fig. 1 isv a graph illustrating the relation between the anode 'current 'and the anode-last dynode voltage of a multiplier tube.

Fig. 2 is a lschematic wiring diagram illustrating one embodiment of the invention.

Fig. 3 is a schematic wiring dagramillustrating another embodiment of the invention.

Generally speaking, in accordance with the present invention an electrostatically focused electron multiplier tube is arranged to receive incident flux from a source of radiant energy. Potentials from a source of substantially constant potential are applied to the electrodes of the multiplier tube through a bleeder resistance. Equi-spaced taps on the bleeder resistance are connected to the cathode of the tube and to the intermediate multiplier electrodes or dynodes thereof. The voltage drop across the bleeder resistance is controlled as a. function of the output current of the multiplier tube. In a logarithmically responsive circuit, the potentials are varied inversely with the output current of the multiplier tube.

According to the present invention, a constant 3 potential means or voltage stabilizer is connected in parallel circuit relation with the multiplier anode and the last multiplier electrode, or #9 dynode as it is frequently designated. Preferably, the current flow through the bleeder resistor is controlled by an electronic amplier tube whose l control grid is connected to the anode of the multiplier tube. 'I'he circuit arrangements are such that the negative bias applied to the ampli'- iier tube control grid increases with an increase in the anode current of the multiplier tube. The output circuit of the amplifier tube is connected in series with the source of constant potential and the bleeder resistor. As the conductivity of the ampliiier tube is a direct function of its control grid potential, the bleeder current will decrease with decreasing control grid potential and thus with increasing multiplier tube anode current. Accordingly, the potential drop across the bleeder resistor is varied inversely with the anode current of the multiplier tube.

A constant potential means, such as a gas iilled voltage regulating tube or a battery, is connected between the cathode of the amplifier tube and the last dynode of the multiplier tube, and in series with the bleeder resistor. The input circuit of the amplifier tube, which thus is connected between the multiplier tube anode and the #9 dynode, is maintained at a constant potential irrespective of the anode current of the multiplier tube. When the arrangement is used as a densitometer, a current measuring meter, preferably a milliammeter,

erence numeral I1 and the last, or #9 dynode, is designated as 30.

The operating potentials applied to cathode I3, anode 25, and dynodes I1 and 30 are obtained from a @suitable voltage divider or bleeder resistor 35, having a relatively negative terminal 2I connected to the negative terminal 22 of a substantially constant vpotential source of direct current having a positive terminal 23. While thesource of power has been indicated as direct current, the invention may also be practiced with alternating current. Direct current has been indicated merely for the purpose of more clearly illustrating the application of the principles of the invention. j

The voltage divider comprises resistors 35 connected in series, with cathode I6 connected to the relatively negative terminal 2I of the series, anode connected to the relatively positive terminal 24 of the series, through the input circuit of a pentode amplifier tube 40, and dynodes Il each connected to a common junction of two adjacent connected resistors. Dynode is connected directly to relatively positive terminal 24.

A conductor 26 connects anode 25 to the control grid 21 of tube 40. A grid biasing resistor 28 is connected from junction point 3| to the positive terminal 32 of constant potential source,

. such as'a battery 33. Battery 33 may be, in a is connected across a small shunt resistor in the amplifier tube output circuit to measure the current flow in the output circuit, and thus provide, on a substantially uniformly graduated scale, direct readings of the density of a sample interposed between a source of light and the multiplier tube.

Referring to the drawing, Fig. 1 is a graph illustrating the relation between the anode current of the multiplier tube and the anode-dynode #9 voltage. Curve lIl represents the anode current as a function of the anode-dynode #9 voltage, with a constant level of illumination on the cathode of the multiplier tube and with a constant voltage applied to the remaining dynodes. In a practical example, at voltages up to approximately fifty volts, the magnitude of the anode current will depend to a considerable extent on the anode voltage. At the higher levels of illumination, the anode current varies only slightly with increasing anode-dynode #9 voltage. Thus, the bleeder resistor current, and consequently the dynode voltages and anode voltage are all quite low. A small change in anode voltage will eiect a very large change of anode current. If the anode-dynode #9 voltage is maintained substantially constant, more particularly at a level above iifty volts, for instance, the bleeder resistor current will vary only slightly with changes in the anode-last dynode voltage, thus rendering the circuit more stable.

One arrangement for accomplishing this result is illustrated in Fig. 2. Referring to Fig. 2, alight a source such as an incandescent lamp I5 is arranged Ato direct light upon the cathode I6 of a photomultiplier tube 20. Photomultiplier tube 20 may, for example, be of the RCA-931 type. Light from lamp I5 is condensed by a lens I I and directed through a filter I2 on to a sample I3 mounted on a suitable support I4.

Photomultiplier tube 20 also includes an anode 25, nine multiplier electrodes or dynodes, of which the rst eight are designated generally by the Icipractical example, either a 90-volt battery or a 90-volt or 105-volt voltage stabilizer tube connected to a conventional source of potential. Screen grid 34 of pentode 40 is likewise connected to positive terminal 32, and anode 36 of the pentode is connected to the positive terminal 23 of the constant potential source;

The potential between multiplier electrode 30 (dynode #9) and anode 25 is controlled by a suitable constant potential means. In the circuit of Fig. 2, a gaseous discharge regulating tube is connected between cathode 31 of pentode 40 and relatively positive terminal 24 of resistors 35, through a small resistor 38. Gaseous discharge tube 45 is preferably one in which the voltage drop therethrough is about iifty-flve volts and is independent of the current flow through the tube. Thus, discharge tube 45 maintains a substantially constant potential of about fty-ve volts between anode 25and #9 dynode 30. 'I'he common `iunction of tube 45 and resistor 38 is connected to one terminal of an indicating meter and to the relatively negative terminal 4I of battery 33. The other terminal of meter 50 is connected to relatively positive terminal 24 of resistor series 35 and through a resistor 42 and an adjustable tap 43 to an intermediate terminal of battery 33. Meter 50 is thus shunted across resistor 38, whereby the potential drop across the resistor, which is a function of the bleeder current of resistors 35, may be measured by meter 50.

The portion of battery 33 between negative terminal 4I and tap 43, in combination with resistor 42, provides a bucking current for meter 50. The reason for this is that, on any uniform scale ammeter used in a densitometer circuit, iniinite density would theoretically yield an innite positive or negative response. For obvious reasons, only a finite range of density can be measured by the meter. Accordingly, a bucking current is introduced into the meter circuit so that the meter will read the maximum measurable density at one extreme of its scale.

The operation of the circuit is as follows. Cathode 31 is maintained at a potential approximately 55 volts positive with respect to terminal 4I of battery 33, due to voltage stabilizer tube 45. The difference in potential between the positive terminal and grid resistor 28, which latter may be approximately 100 megohms, is thus approximately 35 volts. The potential of grid 21 with respectl to cathode 31 will be approximately 35 volts minus the IR drop through resistor 28 due to the anode current of multiplier tube 20. The ohmic value of resistor 28 is so related to the usual values of anode current of multiplier tube 20 that the actual grid-cathode potential of amplifier tube 40 is negative at al1 times in the particular case under consideration, and has a variation of only a few volts under normal operating condition.

The very high positive grid resistor' terminal voltage and the high ohmic value of the grid resistor minimizes the effects of changes in the grid voltage-plate current characteristics of multiplier tube 40. For example, a change of about one-quarter volt in the control grid voltage-plate current characteristic of tube -40 would appreciably change the overall instrument calibration in the circuit described in the above identified application. However, in the present case, such a change would not be serious, as the anode current of multiplier tube 20 would change by only 0.0025 microampere to restore the amplier tube plate-cathode current to its original value.

The density of sample I3 is an inverse logarithmic function of its transmission. The transmission is a direct measure of the light transmitted through the sample from source I5 and incident on. cathode I6 of multiplier tube 20. Thus, the density of sample I3 is an inverse logarithmic function of the light incident upon cathode I6 through the sample.

As set forth in the above lndentied parent application, the anode current of a photomultiplier tube may be varied the proper amount to obtain a logarithmic response if the operating potentials applied to its dynodes are linearly attenuated in accordance With the anode current. This is accomplished by pentode 40 controlling the bleeder current through resistor series 35 in accordance with the anode current of multiplier tube 20'. As the multiplier tube anode current increases, the potential drop across grid 'biasing resistor 28 increases, thus making the control grid 21 more negative with respect to cathode 31. This, in turn, increases the internal resistance of pentode 40 and thus decreases its conductivity. Therefore, the bleeder current from the constant potential sour-ce flowing through resistor series 35 is reduced in proportion to the increase in anode current of multiplier tube 20. The potentials applied between cathode I6, and dynodes I1 and 30 is thus linearly reduced in proportion to the increase in the current from anode 25.

As explained in the copending application, a meter connected to measure the output current of multiplier tube 20, in such as circuit as that described, will provide direct readings of density upon a uniformly graduated scale. Meter 50 is connected across shunt resistor 38 and thus measures the potential drop through the shunt rein operatively low bleeder currents, the potential drop between anode 25 and dynode 30 is not varied disproportionately with respect to its variation at lower levels of illumination resulting in higher bleeder currents.

The anode voltage should not be so high that leakage currentsand other undesired effects are introduced, nor should it be so low that tube 20 is operated on the steep portion of the anode voltage-anode current curve I0 shown in Fig. 1. With the insertion of tube 45 in the cathode-plate circuit of pentode 40, the anode 25-dynode 30 voltage is held at a sensibly constant value irrespective of the. magnitude ofthe bleeder current through resistor series 35. Actually, the value will vary slightly due to the normal change in control grid 27cathode 37 volta'ge of pentode 4f0 and the slight variation in Voltage developed across tube 45 itself over the bleeder current range involved. The adverse eiTects introduced by changes in the control grid voltage-plate cathode current characteristics of pentode 40 do not affect the substantial constancy of the anode potential multiplier tube 20. The use of a grid resistor 28 of high ohmic value and a correspondingly high positive grid |base voltage results in more stable operation than would otherwise be the case. For example, a small change in the control grid voltage producing a given plate current would require a large percentage change in the anode 25 current if the grid bias potential and ohmic value were low. In the present case, where these values are high, such small Voltage change is a negligible fraction of the total bias voltage and the current change thus only a negligible fraction of the total anode 25 current.

Fig. 3 represents a modification of the invention in which a substantially constant potential battery is substituted for gaseous discharge tube 45. In' Fig. 3, elements identical with those in Fig. 2 have been given corresponding reference characters. As shown, a substantially constant potential battery 60 is substituted for voltage stabilizer tube 45, and maintains the potential between anode 25 and dynode 30 at a substantially constant value irrespective of the anode current of tube 20. The .circuit of Fig. 3 operates otherwise in the same manner as does the circuit of Fig. 2.

A thir alternative, which has not been illustrated in order to maintain the drawings as simple as possible, is to make the portion of the bleeder resistor connected between anode 25 and dynode 30, in the manner shown in the above identied application, have an ohmic value so high with respect to that of the rest of resistors 35 that the anode voltage will never drop to a point below about 5I) volts, irrespective of the anode current, over the range of the instrument. This is a level considerably in excess of saturation of multiplier tube 20. However, the latter alternative is less desirable since there will be a very large range of anode voltages involved for a given change in bleeder current values.

The described circuits are substantially completely independent of all ordinary variations in supply line voltage. Additionally, the calibration is unaffected by changes due to the aging of tubes such as pentode 40 and gaseous discharge tube 45, or replacement cf the latter. The only element affecting the calibration is the photomultiplier tube 20. Thus, the overall stability performance of the instrument isincreased.

Furthermore, when the present multiplier tube control circuit is incorporated in a densitorneter for measuring the density of a sample, a stable instrument, having its calibrations substantially unaffected by changes other than changes in the multiplier tube itself, is provided. Such an instrument provides direct readings of density upon a substantially uniformly graduated scale meter. While the circuit is particularly useful in such densitometers, it is not necessarily limited thereto, but is equally applicable to all types of multiplier tubes wherein the output current-anode voltage relationship varies at diiierent rates over a given range of anode voltages.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles thereof, ity will be understood that the invention may be otherwise embodied without departing from such principles.

What is claimed is:

1. An electronic measuring circuit comprising, in combination, an electron multiplier tube including an anode, a cathode and intermediate multiplier electrodes arranged Atherebetween; voltage means connected to the electrodes of said tube for impressing potentials therebetween; a source of potential; variable impedance means connected in electrical series relation with said source and said voltage means and effective to vary the potential drop across said voltage means;

multiplier tube anode current responsive means connected in circuit relation with the output of said tube and with said variable impedance means and effective to vary the impedance of the latter, as a function of such anode current, to modulate such impressed potentials as a function of such anode current; and voltage stabilizing means in circuit connection with said anode and the adiacent multiplier electrode and effective to maintain the potential therebetween substantially constant irrespective of variations in such anode current. 2. An electronic measuring circuit comprising, in combination, an electron multiplier tube including an anode, a cathode and intermediate multiplier electrodes arranged therebetween; bleeder impedance means connected to the electrodes of said Q tube for impressing potentials therebetween; a source of potential; variable impedance means connected in electrical series relation with said source and said bleeder impedance means and effective to vary the potential drop across said bleeder impedance means; multiplier tube anode current responsive means connected in circuit relation -with the output of said tube and with said variable impedance means and eiective to vary the impedance of the latter,

as a function of such anode current, to modulate such impressed potentials as a function of such anode current; and voltage stabilizing means in circuit connection with said anode and the adiacent multiplier electrode and effective to maintain the potential therebetween substantially constant irrespective of variations in such anode current.

3. An electronic measuring circuit comprising,

in combination, an electron multiplier tube inand with said variable impedance means and effective to vary the impedance of the latter, as a function of such anode current, to modulate such 'i impressed potentials as a function of such anode current; and voltage stabilizing means in circuit .connection with said anode and lthe adjacent multiplier electrode and effective to maintain the potential therebetween substantially constant irrespective of variations in such anode current.

4. An electronic measuring circuit comprising, in combination, an electron multiplier tube including an anode, a cathode and intermediate multiplier electrodes arranged therebetween; voltage means connected to the electrodes of said tube for impressing potentials therebetween; a source of potential; variable impedance means connected in electrical series relation with said source and said voltage means and effective to vary the potential drop across said voltage means; multiplier tube anode current responsive means connected in circuit relation with the output of said tube and with said variable impedance means and effective to vary the impedance of the latter, as a function of such anode current, to modulate such impressed potentials as a function oi' such anode current; and voltage stabilizing means in parallel circuit connection with said anode and the adjacent multiplier electrode and in series circuit relation with said variable impedance means and effective to maintain the potential `between said anode and said adjacent multiplier electrode substantially constant irrespective of variations in such anode current.

5. An electronic measuring circuit comprising, in combination, an electron multiplier tube including an anode, a cathode and intermediate multiplier electrodes arranged therebetween; bleeder impedance means connected to the electrodes of said tube for impressing potentials therebetween; a source of potential; variable impedance means connected in electrical series relation with said source and said bleeder impedance means and effective to Vary the potential drop across said bleeder impedance means; multiplier tube anode current responsive means connected in circuit relation with the output of said tube and with said variable impedance means and effective to vary the impedance of the latter, as a function of such/anode current, to modulate such impressed potentials as a function of such anode current; and voltage stabilizing means in parallel circuit connection with said anode and the adjacent multiplier electrode and in series circuit relation with said variableimpedance means and effective to maintain the potential between said anode and said adjacent multiplier electrode substantially constant irrespective of variations in such anode current.

6. An electronic measuring circuit comprising, in combination, an electron multiplier tube includingan anode, a cathode and intermediate multiplier electrodes arranged therebetween; bleeder resistor means connected to the electrodes of said tube forl impressing potentials therebetween; a source of potential; variable impedance means connected in electrical series relation with said source and said bleeder resistor means and eiective to vary the potential drop across said bleeder resistor means; vmultiplier tube anodi current responsive means connected in circuit relation with the output of said tube and with said variable impedance means and effective to vary the impedance of the latter, as a function of such anode current, to modulate such impressed potentials as a function of such anode current; and voltage stabilizing means in parallel circuit connection with said anode and the adjacent multiplier electrode and in series circuit rela-tlon with said variable impedance means and effective to maintain the potential between said anode and said adjacent multiplier electrode substantially constant irrespective of variations in such anode current.

7. An electronic measuring circuit comprising, in combination, an electron multiplier tube including an anode, a .cathode and intermediate multiplier electrodes arranged therebetween; voltage means connected to the electrodes of said tube for impressing potentials therebetween; a source of potential; a thermionic amplifier tube having its output connected in electrical series relation with said source and said voltage means and effective to vary the potential drop across said voltage means; multiplier tube anode current responsive means connected in circuit relation with the output of said multiplier tube and with the input of said amplifier tube and effective to vary the amplification factor of the latter, as a function of such anode current, to modulate such impressed potentials as a function of such anode current; and voltage stabilizing means in circuit connection with said anode and the adjacent multiplier electrode and effective to maintain the potential therebetween substantially constant irrespective of variations in such anode current.

8. An electronic measuring circuit comprising, in combination, an electron multiplier tube including an anode, a cathode and intermediate multiplier electrodes arranged therebetween; bleeder impedance means connected to the electrodes of said tube for impressing potential therebetween; a source of potential; a thermionic amplifier tube having its output connected in electrical series relation with said source and said bleeder impedance means and effective to vary the potential drop across said bleeder impedance means; multiplier tube anode current responsive means connected in circuit relation with the output of said multiplier tube and with the input of said amplifier tube and effective to vary the amplification factor of the latter, as a function of such anode current,I to modulate such impressed potentials as a function of such anode current; and voltage stabilizing means in circuit connection Witii said anode and the adjacent multiplier electrode and effective to maintain the potential therebetween substantially constant irrespective of variations in such anode current.

9. An electronic measuring circuit comprising, in combination, an electron multiplier tube including an anode, a cathode and intermediate multiplier electrodes arranged therebetween; bleeder resistor means connected to the electrodes of said tube for impressing potentials therebetween; a source of potential; a thermionic amplier tube having its output connected in electrical series relation withsaid source and said bleeder resistor means and effective to vary the potential drop across said bleeder resistor means; multiplier tube anode current responsive means connected in circuit relation with the output of said tube and with the input of said amplifier tube and effective to vary the amplification factor of the latter, as a function of such anode current, to modulate lsuch impressed potentials as a function of such anode current; and voltage stabilizing means in circuit connection with said anode and the adjacent multiplier electrode and effective to maintain the potential therebetween l0r` substantially constant irrespective of variations in such anode current.

10. An electronic measuring circuit comprising, in combination, an electron multiplier tube including an anode, a, cathode and intermediate multiplier electrodes arranged therebetween; voltage means connected to the electrodes of said tube for impressing potentials therebetween; a

source of potential; a thermionic amplifier tubev having its output connected in electrical series relation with said source and said voltage means and effective to vary the potential drop across said voltage means; multiplier tube anode current responsive means connected in circuit relation with the output of said multiplier tube and with the input of said amplifier tube and effective to vary the amplification factor of the latter, as a function of such anode current, to modulate such impressed potentials as a function of such anode current; and voltage stabilizing means in parallel circuit connection with said anode and the adjacent multiplier electrode and -in series circuit relation with the output of said amplifier tube and effective to maintain the potential between said anode and said adjacent multiplier elec-trede substantially constant irrespective of variations Iin such anode current.

11. An electronic measuring circuit comprisin-g, in combination, an electron multiplier tube including an anode, a cathode and intermediate multiplier electrodes arranged therebetween; bleeder impedance means connected to the electrodes of said tube for impressing potential therebetween; a source of potential; a thermionic amplifier tube having its output connected in electrical series relation with said source and said bleeder impedance means and effective to vary the potential drop across said bleeder impedance means; multiplier tube anode current responsive means connected in circuit relation with the output of said multiplier tube and with the input of said amplier tube and effective to vary the amplification factor of the latter, as a function of such anode current, to modulate such impressed potentials as a function of such anode current; and vol-tage stabilizing means in parallel circuit connection with said .anode and the adjacent multiplier electrode and in series ycircuit relation with lthe output of said amplifier tube and eiective to maintain the potential betweensaid anode and said adjacent multiplier electrode substantially constant irrespective of variations in such anode current.

12. An electronic measuring circuit comprising, in combination, an electron multiplier tube including an anode, a cathode and intermediate multiplier electrodes arranged therebetween; bleeder resistor means connected to the electrodes of said tube for impressing potentials therebetween; a source of potential; a thermionic amplier tube having its output connected 'in electrical series relation with said source and said bleeder resistor means and effective to vary the potential drop across said bleeder resistor means; multiplier tube anode current responsive means connected in circuit relation with the output of said tube and with the input of said amplifier tube and effective to Vary the amplification factor of .the latter, as a function of such anode current, to modulate such impressed potentials as a function of such anode current; and voltage stabilizing means in parallel circuit connection with said anode and the adjacent multiplier electrode and in series circuit relation with the output of said amplier tube and effective to maintain the potential between said anode and said adjacent multiplier electrode substantially constant irrespectlve of Variations in such anode current.

13. A logarithmically responsive electronic measuring circuit comprising, in combination, lan electron multiplier tube including an anode, a cathode and intermediate multiplier electrodes arranged therebetween; voltage means connected to the electrodes of said tube for impressing potentials therebetween; a source of potential; variable impedance means connected in electrical series relation w-tih said source and said voltage lmeans and effective to vary the potential` drop across said voltage means; multiplier tube anode current responsive means connected in circuit relation with the output of said tube Iand with said variable impedance means and effective to vary the impedance of lthe latter, as an inverse function of such anode current, to modulate such impressed potentials as an inverse function of such anode current; and voltage stabilizing means in circuit connection with said anode and the adjacent multiplier electrode and effective to maintain the potential therebetween substantial- 1y constant irrespective of variations in such anode current.

14. A logarithmically responsive electronic measuring circuit comprising, in combination, an electron multiplier tube including an anode, a cathode and intermediate multiplier electrodes arranged therebetween; bleeder resistor means connected to the electrodes of said tube for impressing potentials therebetween; a source of potential; a thermionic amplifier tube having its output connected in electrical series relation with said source and said bleeder resistor means and eiective to vary the potential drop across said bleeder resistor means;' multiplier tube anode current responsive means connected in circuit relation with the output of said multiplier tube and with the input of said amplifier tube and effective to vary the amplification factor of the latter, as an inverse function of such anode current, to modulate such impressed potentials as an inverse function of such anode current; and voltage stabilizing means in circuit connection with said anode and the adjacent multiplier electrode and effective to maintain the potential therebetween substantially constant irrespective of variations in such anode current.

15. A logarithmically responsive electronic measuring circuit comprising, in combination, an electron multiplier tube including an anode, a cathodel and intermediate multiplier electrodes arranged therebetween; voltage means connected to the electrodes of said tube for impressing potentials therebetween; a source of potential; a thermionie amplifier tube having its output connected in electrical series relation with said source and said voltage means and effective to vary the potential drop across said voltage means; multiplier tube anode current responsive means,

. including a conductor connecting said anode to the control grid of said amplifier tube, eiective to vary the amplification factor of the latter, as an inverse function of such anode current, to modulate such impressed potentials as an inverse function of such anode current; and voltage stabilizing means in parallel circuit connection with said anode and the adjacent multiplier electrode and effective to maintain the potential therebetween substantially constant irrespective of variations in such anode current.

16. A logarithmically responsive electronic measuring circuit comprising, in combination, an

electron multiplier tube including an anode, a cathode and intermediate multiplier electrodes arranged therebetween; bleeder resistor means connected to the electrodes 0f said tube for lmpressing potentials therebetween; a source of potential; a thermionic amplifier tube having its output connected in electrical series relation with said source and said bleeder resistor and eiective to vary the potential drop across said bleeder resistor; multiplier tube anode current responsive means, including a conductor connecting said anode to the control grid of said amplifier tube, eiective to vary the amplification factor of the latter, as an inverse function of such anode current, to correspondingly vary the bleeder current to modulate such impressed potentials as an inverse function of such anode current; and voltage stabilizing means in parallel circuit connection with said anode and the adjacent multiplier electrode and eective to maintain the potential therebetween substantially constant irrespective of variations in such anode current.

17. A logarithmically responsive electronic measuring circuit comprising, in combination, an electron multiplier tube including an anode, a cathode and intermediate multiplier electrodes arranged therebetween; a bleeder resistor having one terminal connected to said cathode and the other terminal connected to the multiplier electrode adjacent said anode; circuit means connecting equi-spaced points on said resistor to the other multiplier electrodes; a thermionic amplifier tube having a cathode connected to said other resistor terminal for controlling the bleeder current through said resistor to modulate the potentials impressed on the elements of said multiplier tube; a source of potential; means connecting one terminal of said source to said one resistor terminal and the other terminal of said source to the plate of said amplifier tube; multiplier tube anode current responsive means, including a conductor connecting said anode to the control grid of said amplier tube, effective to vary the amplification factor of the latter. as an inverse function of such anode current, to correspondingly vary the bleeder current to modulate such impressed potentials as an inverse function of such anode current; and voltage stabilizing means in circuit connection with said anode and the adjacent multiplier electrode and effective to maintain the potential therebetween substantially constant irrespective of variations in such anode current.

18. A measuring circuit comprising, in combination, an electron multiplier tube including an anode, a cathode and intermediate multiplier electrodes arranged therebetween; a bleeder resistor having one terminal connected to said cathode and the other terminal connected to the multiplier electrode adjacent said anode; circuit means connecting equi-spaced points on said resistor to the other multiplier electrodes; a thermionic amplifier tube having a cathode connected to said other resistor terminal for controlling the bleeder current through said resistor to modulate the potentials impressed on the elements of said multiplier tube; a source of potential; means connecting one terminal of said source to said one resistor terminal and the other terminal of said source to the plate of said amplifier tube; a grid biasing potential source having one terminal in circuit connection with the cathode of said amplifier tube and the other terminal connected, in series with a grid biasing resistor, to the control grid of said amplifier tube; a conductor connecting said anode logarithmically responsive electronicto the control grid in Parallel with said biasing resistor, whereby the amplification factor of said amplifier tube will be varied as an inverse function of the multiplier tube anode current to, in turn, vary the bleeder current to modulate such impressed potentials as an inversev function of suchanode current; and voltage stabilizing means connected in series circuit relation with said other resistor terminal and the amplifier tube cathode to maintain the potential between said anode and such adjacent multiplier electrode substantially constant irrespective of variations in such anode current, to stabilize the operation of said circuit.

19. A logarithmicaliy responsive electronic measuring circuit comprising, in combination, an electron multiplier tube including an anode, a cathode and intermediate multiplier electrodes arranged therebetween; a bleeder resistor having one terminal connected to said cathode and the other terminal connected to the multiplier electrode adjacent said anode; circuit means connecting equi-spaced points on said resistor to the other multiplier electrodes; a thermionic amplifier tube having a cathode connected to said other resistor terminal for controlling the bleeder current through said resistor to modulate the potentials impressed on the elements of said multiplier tube; a source of potential; means connecting one terminal of said source to said one resistor terminal and the other terminal of said source to the plate of said amplifier tube; a grid biasing potential source having one terminal in circuit connection with the cathode of said amplifier tube and the' other terminal connected, in series with a grid biasing resistor, to the control grid of said amplifier tube; a conductor connecting said anode to the control grid in parallel with said biasing resistor, whereby the amplification factor of said amplifier tube will be varied as an inverse function of the multiplier tube anode current to, in turn, vary the bleeder current to modulate such impressed potentials as an inverse function of such anode current; and a gaseous discharge voltage stabilizing tube connected in series circuit relation with said other resistor terminal and the ampliiier tube cathode to maintain the potential between said anode and such adjacent multiplier electrode substantially constant irrespective of variations in su'ch anode current, to stabilize the operation of said circuit.

20. A logarithmically responsive electronic measuring circuit comprising, in combination, an electron multiplier tube including an anode, a cathode and intermediate multiplier electrodes arranged therebetween; a bleeder resistor having one terminal connected to said cathode and the other terminal connected to the multiplier electrode adjacent said anode; circuit means connecting equi-spaced points on said resistor to the other multiplier electrodes; a thermionic ampliiier tube having a cathode connected to said other resistor terminal for controlling the bleeder current through said resistor to modulate the potentials impressed on the elements of said multiplier tube; a source of potential; means connecting one terminal of said source to said one resistor terminal and the other terminal of said source to the plate of said amplifier tube; a grid biasing potential source having one terminal in circuit connection with the cathode of said ampliiler tube and the other terminal connected, in series with a grid biasing resistor, to the control grid of said ampliiier tube; a conductor connecting said anode to the control grid in parallel with said biasing resistor, whereby the amplification factor of said amplier tube will be varied as an inverse function of the multiplier tube anode current to, in turn, vary the bleeder current to modulate such impressed potentials as an inverse function of such anode current; and a battery connected in series circuit relation with said other resistor terminal and the ampliiier tube cathode to maintain the potential between said anode and such adjacent multiplier electrode substantially constant irrespective of variations in such anode current, to stabilize the operation of said circuit.

MONROE H. SWEET.

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